The present invention relates to a method of manufacturing an image sensor for use in facsimile machines or scanners, and particularly to a method of manufacturing, under a simple process, an image sensor, having a group of matrix-formed wiring lines, capable of outputting read signals correctly.
In a conventional image sensors, particularly a contact type image sensor, image information of a document, for example, is projected onto the sensor in one-to-one correspondence, and then the sensor converts the image information into electrical signals. In this case, the projected image is divided into a multiplicity of pixels (light-receiving elements) and the electric charge generated at the respective light-receiving elements is temporarily stored at interline capacitors of a group of wiring for every predetermined block using a thin film transistor switching element (TFT). Then, the stored electric charge is sequentially read as electric signals at speeds ranging from several hundreds of KHz to several MHz. Such a TFT-driven image sensor can read image information with a single drive IC owing to TFT operation, thereby contributing to reducing the number of drive ICs, which serve to drive the image sensor.
The TFT image sensor, e.g., as shown in FIG. 7, includes a line shaped light-receiving element array 11 whose length is almost the same as the length of a document; an electric charge transfer section 12 consisting of a plurality of thin film transistors Ti,j (i=1 to N, j=1 to n) corresponding to the respective light-receiving elements on a one-to-one basis; and a group of wiring lines 13.
The light-receiving element array 11 consists of a plurality (N) of blocks of light-receiving elements, a plurality (n) of light-receiving elements 11a, which constitute each block, can be represented equivalently as a plurality of photodiodes PDi,j (i=1 to N, j=1 to n). Each light-receiving element 11a is connected to the drain electrode of each thin film transistor Ti,j. The source electrode of each thin film transistor Ti,j is connected to each of a plurality (n) of common signal lines 14 for every block of light receiving elements through the matrix-formed wiring group 13. The gate electrode of each thin film transistor Ti,j is connected to a gate pulse generating circuit (not shown) so that the thin film transistors Ti,j are turned on for every block. The photoelectric charge generated at each light-receiving element 11a is temporarily stored at both a parasitic capacitor of each light-receiving element and an overlap capacitor arranged between the drain and gate of the thin film transistor and thereafter, sequentially transferred to and stored in the interline capacitors Cj of the wiring group 13 for every block using each thin film transistor Ti,j as an electric charge transfer switch. Specifically, a gate pulse .PHI.G1 from the gate pulse generating circuit turns on the thin film transistors T1,1 to T1,n, thereby transferring to and storing the electric charge generated and stored at each light-receiving element 11a in the first block to each interline capacitor Cj. The electric charge stored at each interline capacitor Cj changes the potential of each common signal line 14, and each changed potential is taken out at an output line 16 by sequentially turning on analog switches SWn within a drive IC 15. The thin film transistors T2,1-T2,n to TN,1-TN,n of the second to Nth blocks are similarly turned on by gate pulses .PHI.G2 to .PHI.Gn, transferring the electric charge on the side of the light-receiving elements for every block. By sequentially reading the transferred electric charge, pixel signals equivalent to a single line in a main scanning direction of the document can be obtained. The above operation is repeated by shifting the document by document forwarding means (not shown) such as rollers, thereby obtaining the pixel signals of the entire document. Japanese Patent Unexamined Publications Nos. 9358/1988 and 67772/1988 provide a description relating to this type of operation.
The matrix-formed wiring group 13 is generally arranged as follows. It includes, as shown in FIG. 8 by its sectional view, lower wiring 31, an insulating layer 33, upper wiring 32, all of these being sequentially formed on a substrate 21. Both the lower wiring 31 and the upper wiring 32 are arranged so as to intersect orthogonally, and contact holes 34 are provided to connect both upper and lower wiring lines.
To overcome the problem of crosstalk generated at each intersecting portion of the lines in the matrix-formed wiring group 13, an insulating layer 33a, a ground sheet 35 which is connected to a ground line, and an insulating layer 33b are arranged to prevent the crosstalk at the ground sheet 35 as shown in FIG. 9. Japanese Patent Unexamined Publication No. 67864/1987provides a description relating to this type of arrangement. The crosstalk is a phenomenon that since a capacitor exist at a portion where signal lines intersect in a multilayer form, a change in the potential of one signal line is transferred to another signal line through the capacitor and causes a change in the potential of another.
However, the wiring line group having the ground sheet not only induces large parasitic capacitors between the wiring lines and the ground sheet but also bends the entire image sensor due to warpage of the ground sheet. Thus the ground sheet has been modified as shown in FIG. 10 by its sectional view. Specifically, an intermediate ground wiring layer 36 using a meshed ground member is provided around each portion where the upper and lower wiring lines intersect. Japanese Patent Unexamined Publication No. 5057/1989 provides a description relating to this type of arrangement.
However, the method of manufacturing an image sensor in which the light-receiving elements, the thin film transistor switching elements, and the wiring line group including the meshed intermediate ground wiring layer are formed simultaneously and efficiently on a single substrate has not been proposed. Thus the conventional fabricating process has been complicated.